This invention relates generally to integrated circuit processes and fabrication, and more particularly, to a system and method of adhering copper to a diffusion barrier surface.
The demand for progressively smaller, less expensive, and more powerful electronic products, in turn, fuels the need for smaller geometry integrated circuits (ICs), and large substrates. It also creates a demand for a denser packaging of circuits onto IC substrates. The desire for smaller geometry IC circuits requires that the interconnections between components and dielectric layers be as small as possible. Therefore, research continues into reducing the width of via interconnects and connecting lines. The conductivity of the interconnects is reduced as the surface area of the interconnect is reduced, and the resulting increase in interconnect resistivity has become an obstacle in IC design. Conductors having high resistivity create conduction paths with high impedance and large propagation delays. These problems result in unreliable signal timing, unreliable voltage levels, and lengthy signal delays between components in the IC. Propagation discontinuities also result from intersecting conduction surfaces that are poorly connected, or from the joining of conductors having highly different resistivity characteristics.
There is a need for interconnects and vias to have both low resistivity, and the ability to withstand volatile process environments. Aluminum and tungsten metals are often used in the production of integrated circuits for making interconnections or vias between electrically active areas. These metals are popular because they are easy to use in a production environment, unlike copper which requires special handling.
Copper is a natural choice to replace aluminum in the effort to reduce the size of lines and vias in an electrical circuit. The conductivity of copper is approximately twice that of aluminum and over three times that of tungsten. As a result, the same current can be carried through a copper line having half the width of an aluminum line.
The electromigration characteristics of copper are also much superior to those of aluminum. Copper is approximately ten times better than aluminum with respect to electromigration. As a result, a copper line, even one having a much smaller cross-section than an aluminum line, is better able to maintain electrical integrity.
There have been problems associated with the use of copper, however, in IC processing. Copper pollutes many of the materials used in IC processes and, therefore, care must be taken to keep copper from migrating. The migration of copper into silicon semiconductor regions is especially harmful. The conduction characteristics of the semiconductor regions are a consideration in the design of a transistors. Typically, the fabrication process is carefully controlled to produce semiconductor regions in accordance with the design. Elements of copper migrating into these semiconductor regions can dramatically alter the conduction characteristics of associated transistors.
Various means have been suggested to deal with the problem of copper diffusion into integrated circuit material. Several materials, especially metallic ones, have been suggested for use as barriers to prevent the copper diffusion process. For example, Cho et al., in the article entitled "Copper Interconnection with Tungsten Cladding for ULSI," 1991 Symposium on VLSI Technology, pg. 39, suggests the use of tungsten as a diffusion barrier. Molybdenum and titanium nitride (TiN) have also been suggested for use as copper diffusion barriers. Gardner, et al., in an article entitled "Encapsulated Copper Interconnection Devices Using Sidewall Barriers," in 1991 VMIC Conference, pg. 99, suggests the use of sidewall structures to completely encapsulate the copper. However, the adhesion of copper to these diffusion barrier materials has been, and continues to be, an IC process problem.
Copper cannot be deposited onto substrates using the conventional processes for the deposition of aluminum when the geometries of the selected IC features are small. That is, new deposition processes have been developed for use with copper in the lines and interconnects of an IC interlevel dielectric. It is impractical to sputter metal, either aluminum or copper, to fill small diameter vias, since the gap filling capability is poor. To deposit copper, a chemical vapor deposition (CVD) technique has been developed in the industry.
In a typical CVD process, copper is combined with a ligand, or organic compound, to make the copper volatile. That is, copper becomes an element in a compound that is vaporized into a gas. Selected surfaces of an integrated circuit, such as difflusion barrier material, are exposed to the copper gas in an elevated temperature environment. When the copper gas compound decomposes, copper is left behind on the selected surface. Several copper gas compounds are available for use with the CVD process. It is generally accepted that the configuration of the copper gas compound, at least partially, affects the ability the copper residue to adhere itself to the selected surface.
Wang, et al. in the article "Chemical Mechanical Polishing of Copper Metalized Multi-level Interconnection Devices," 1995 VMIC Conference, pg. 505, suggests the use of one particular copper gas compound, or precursor, for improving the adhesion of copper to a TiN barrier surface. Although certain precursors may improve the copper adhesion process, variations in the diffusion barrier surfaces to which the copper is applied, and variations in the copper precursors themselves, often result in the unsatisfactory adhesion of copper to a selected surface.
It has become standard practice in the industry to apply CVD copper immediately after the deposition of the diffusion barrier material to the IC. Typically, the processes are performed in a single chamber or an interconnected cluster chamber. It has generally been thought that the copper layer has the best chance of adhering to the diffusion barrier material when the diffusion barrier material surface is clean. Hence, the diffusion barrier surface is often kept in a vacuum, or controlled environment, and the copper is deposited on the diffusion barrier as quickly as possible. However, even when copper is immediately applied to the diffusion barrier surface, problems remain in keeping the copper properly adhered. A complete understanding of why copper does not always adhere directly to a diffusion barrier surface is lacking.
It would be advantageous to employ a method of improving the adhesion of CVD copper to a diffusion barrier material surface.
It would also be advantageous if a method were employed for preparing a diffusion barrier surface, in advance of CVD copper deposition, to improve the adhesion of copper of the diffusion barrier surface.
Further, it would be advantageous if the adhesion improving process did not degrade the electrical conductivity between the deposited copper and a conductive diffusion barrier material. It would also be advantageous if the process did not disrupt the silicon bonds and structures in adjoining IC substrates.
Accordingly, a method of applying copper to selected integrated circuit surfaces is provided. The selected copper-receiving surfaces being predominantly on diffusion barrier material applied to selected regions of the IC. The method comprises the steps of: exposing each selected copper-receiving surface to a reactive gas species; permitting the reactive gas species to interact with molecules of the copper-receiving surface, whereby higher energy molecular bonds are replaced with lower energy bonds between the reactive gas species and the exposed copper receiving surface, changing the surface characteristics of the exposed copper-receiving surface to promote the formation of bonds between the copper-receiving surface and subsequently deposited copper; and depositing CVD copper on the copper-receiving surface, whereby copper adhesion to the diffusion barrier is improved.
In a preferred embodiment of the invention, the method includes generating the reactive gas species from a plasma source. The method also includes using a direct plasma source having a radio frequency (RF) power level of less than approximately 500 watts to generate the reactive gas species, whereby the relatively low energy level of the plasma ions minimizes the disruption of silicon crystalline structures.
It is a feature of the invention that the reactive gas species is selected from the group consisting of O.sub.2, H.sub.2, N.sub.2, Ar, He, Ne, Kr, Xe, and volatile C compounds. The volatile C compound is selected from the group consisting of CO, CH.sub.4, C.sub.2 H.sub.6, C.sub.2 F.sub.6, C.sub.3 F.sub.8, C.sub.4 F.sub.8, CHF.sub.3, CH.sub.3 F, and CF.sub.4.
In a preferred embodiment of the invention the diffusion barrier material is selected from the group consisting of TiN, TiON, TiSiN, TaSiN, TaN, TiW, TiWN, Mo, and WN, whereby copper is adhered to a copper-receiving surface which permits electrical communication between the copper and regions of the IC underlying the copper-receiving surface. It is also a feature of the invention that the diffusion barrier material is selected from the group consisting of BN, Si.sub.3 N.sub.4, and SiBN, whereby copper is adhered to an electrically insulating copper-receiving surface.
An adherent copper conductor interface on an integrated circuit is also provided, comprising a semiconductor layer, and a copper layer overlying the semiconductor layer. The adherent copper conductor interface also comprises a diffusion barrier between the semiconductor layer and the copper layer with a surface adjoining the copper layer, the diffusion barrier surface being exposed to a reactive gas species to permit the reactive gas species to interact with molecules of the diffusion barrier surface, changing the surface characteristics of the exposed diffusion barrier surface to promote the formation of bonds between the exposed diffusion barrier surface and the subsequently deposited copper layer.
In a preferred embodiment of the invention the diffusion barrier is TiN. It is a feature of the invention that the diffusion barrier surface is prepared for adhesion to the copper layer with a reactive hydrogen species. Typically, the reactive hydrogen species is generated from a plasma source.
A co-pending application Ser. No. 08/717,267 now U.S. Pat. No. 5,913,144, filed Sep. 20, 1996, entitled, "Oxidized Diffusion Barrier Surface for the Adherence of Copper and Method for Same", invented by Tue Nguyen, Lawrence J. Charneski, and Lynn R. Allen, which is assigned to the same assignees as the instant patent, discloses a method for oxidizing the diffusion barrier surface to improve the adherence of copper to the diffusion barrier.
It has been standard practice in the industry to keep a diffusion barrier surface, located on a selected surface of an IC, in a controlled environment whenever possible, and to apply the copper as quickly as possible. This practice is based on the belief that protecting the diffusion barrier from uncontrolled gas environments, and keeping the barrier clean, provides the best foundation for copper adhesion. However, as demonstrated in the present invention, the preparation of the copper-receiving surface with a reactive gas species prepares the copper-receiving surface to adhesion to a copper layer by roughing the surface to increase the total area of the surface available for bonding to the copper. In addition, elements of the diffusion barrier surface bond to the reactive gas species. These bonds are replaced, relatively easily, by copper so that bonds to the diffusion barrier surface are formed, improving adhesion. Alternately, the reactive gas species disrupts molecular bonding along the surface of the diffusion barrier creating high energy, unstable, bonds. Copper deposited on the diffusion barrier surface breaks these unstable bonds to form stable bonds to the diffusion barrier surface, which improves copper adhesion.
The low power levels and temperatures required to perform this process ensure that minimum damage is done to the associated substrates in the integrated circuit. Since the plasma exposure process is generally completed in less than 60 seconds, a minimum of damages done to the IC crystalline structures and a speedy, commercially viable, process is ensured. The improved adhesion resulting from exposure to a reactive gas species permits a greater degree of variation in the uniformity of the diffusion barrier surface and the precursor. Further, the diffusion barrier deposition process and the copper deposition processes can be carried out in different chambers, and at different times, because of the reduced concern over the cleanliness of the process diffusion barrier surface.